This invention relates to coupled resonator filter oscillators. More particularly this invention relates to an oscillator having a resonator filter as a finite impulse response, a filter as the frequency control element and a balanced quadrature amplifier as a sustaining device.
Prior art oscillators use a two pole-pair resonator SAW filter as a frequency control element coupled to an amplifier in an inductorless oscillator circuit. The performance of the prior art oscillators, however, is too imprecise for critical applications, since the oscillator frequency can be anywhere in the resonator filter passband. More demanding applications, requiring outstanding spectral purity and/or frequency accuracy, must maintain better control of the filter to amplifier interface impedance. An impedance mismatch between the filter and the amplifier will distort the filter's transfer function and frequently results in excessive insertion loss.
There is no reason to constrain the filter in an oscillator to two poles. In fact, superior performance can be obtained with four pole-pairs for variable frequency oscillators and six pole-pairs for fixed frequency oscillators. For a given bandwidth, higher order filters have a much steeper phase slope, greatly improving an oscillator's spectral purity.
In the case of acoustic wave guide coupled resonator filters, multiple pole-pairs can be cascaded without the need for intervening matching networks.
Using high order filters to their best effect requires good control of excess oscillator loop gain. It is essential that the filter's magnitude versus frequency characteristic has reduced the oscillator loop gain to less than unity before the loop phase curve versus frequency has reached a second point of positive feedback (commonly referred to as a .pi. point).
Also, for other important reasons, excess oscillator loop gain is detrimental to phase noise performance. There should be no more loop gain in the oscillator than is required to start and maintain oscillations under all conditions of temperature, aging, supply voltage variations, and the like.
If the resonator filter is synthesized to represent an approximation to linear phase in the passband (e.g. Bessel, transitional Butterworth-Thomson, equiripple phase, etc.) it can be an excellent frequency control element for variable frequency oscillators, since a linear tuning curve is highly desirable.
As noted earlier, resonator filters, like their electric-wave (having inductors and capacitors) counterparts, require a well defined source and load impedance. Indeed, the source and load impedance are an integral and inseparable part of the filter design. If the filter terminating impedance is changed, virtually all elements in the filter have to be changed to preserve the filter's original transfer characteristic. Any deviation from the filter's design source and load will distort the desired magnitude and phase characteristics of the oscillator. Clearly, precise filter source and load impedance control is necessary to realize the full potential of the resonator oscillator.
Without the addition of some corrective mechanism, the fundamental frequency terminal impedances of the amplifier changes due to amplifier internal power limiting as oscillations build from a small signal to large signal conditions. In short, in a conventional oscillator, the amplifier's input and output impedances are not time invariant, a fact that violates the design supposition of the filter (and other passive circuits in the loop).
Another problem with prior art oscillators is that it is difficult to satisfy the filter, transistor noise figure, gain, and output impedance matching criteria simultaneously. Furthermore, it has been found that it is preferable to use negative feedback in trimming the open loop excess gain. Negative feedback changes both the input and output impedance of the amplifier, representing a further complication in design and manufacturing.
As a practical matter, the oscillator gain and tuning adjustments should be made open loop. Further, the open loop impedance should match the impedance when the loop is closed. Obviously, it would be preferable if the oscillator's impedance could be measured with commonly used test instruments. Again, the change in amplifier impedance precludes accurate tuning and adjustment.
As the amplifier's signal parameters change, it induces insertion phase changes in the other oscillator circuit elements, creating AM/PM conversion, which in turn can degrade the oscillator's spectral purity.
Therefore, a need exists for an improved oscillator wherein the amplifier represents a fixed, time invariant, terminating impedance for both small and large signal conditions. Furthermore, impedance changes due to amplifier adjustments for gain, noise figure, output power, and the like, should be isolated from the frequency control element and other circuits, such as the phase shifter and signal coupling circuits.